Multi-stage axial-flow compressors with adjustable pitch stator blades



Apnl 5, 1955 A. c. LOVESEY ET AL 2,705,590

MULTI-STAGE AXIAL-FLOW COMPRESSORS WITH ADJUSTABLE PITCH STATOR BLADES Filed Oct. 15, 1950 4 Sheets-Shet 1 W x 5 g W I Q 7 INVEN OM AC. LOVESEY, N a L. WILD)? D. 0. DAV/ES April 5, 1955 A. c. LOVESEY ET AL 2,70

MULTI-STAGE AXIAL-FLOW COMPRESSORS WITH ADJUSTABLE PITCH STATOR BLADES 4 Sheets-Sheet 2 Filed Oct. 13, 1950 INVENTORS c. LO VESE G: -L. WILDE D. O. DAVIE 5 April 5, 1955 Fild Oct. 13, 1950 A. C. LOVESEY ET AL MULTI-STAGE AXIAL-FLOW COMPRESSORS WITH ADJUSTABLE PITCH STATOR BLADES 4 Sheets-Sheet 5 a. 4. w/zM/ 0. o. m was MMM April 5, 1955 'A. c. LOVESEY ETAL 2,705,590

MULTI-STAGE AXIAL-FLOW COMPRESSORS WITH ADJUSTABLE PITCH STATOR BLADES Filed 001;. 13, 1950 4 Sheets-Sheet 4 y Q0 DAVIES United States Patent MULTI-STAGE AXIAL-FLOW GOMPRESSQRS WITH AB X T H TAT Q BLADE Alfred Cyril Lovesey, Littleover, Geoffrey Light Wilde,

Coxhench, and David Omri Davies, Edgware," England, England, a

assignors 'to, Rolls-Royce LimitedQDerby,

lrfi ish qmr ny Application October, 13, 1950, Serial No..18?,9 68

Claims. priority, application Great Britain October 28, 1949 11 C i s (or 23 x114) the axial; velocity of the fluid and the direction of, swirl given to the fluid by the stator blades.

At speeds below the" designed speed, when; the pres;

spre rise per stage is less than the designed ptessure rise, there will tend to be an accelerating tlpwl from the entry. to the exit of the compressor due to the designed over-all density ratio not being achieved. This acceleration appears as a reduction in; the axial velocity. at the entry and an increase in the axial velocity at the exit.

Where the axial velocity; of thefluid is lower than the designed axial vlocity,'the angle of incidence becomes positive and so high that the bla'desrun in a st'alled condition, and where t designed velocity, the blades runat a negative angle of incidence and therefore do no contribute. theif share. to the pressure rise. At. speeds above the. desighed speed, when the pressure rise per stage i'sjgreater than madesigned pressure rise, there"willfltendr to beat decelerating fio'yjvfrom the entryofithe compressor to the exodus to the designed over-all. density ratio being exceeded. This deceleration appears. as an increase of the axialvelocit'y.

over the. designed velocity at the 'entryfand se r-aros at the exit.

Therefore any departure of the operating. condition of the axial-flow Compressor, fromthe 'de sig'iied speed'or from the condition dependent onthe 'speed. adversely effects the elficiency of the compressor.

To enable a multistage, axial flow compressor. to, opcrate. with high efiiciencyover a range of compressor. rotational speedsithas been proposed toprovide adjustable pitch stator blades, and a, multi-Istage axial-flow com ressor having such an arrangement of. stator, blades 1s described and claimedfin U. 5. Patent No. '2,6l3 ,0 29

The'term rotational speed is to be nhdei stood'as meaning either the actual rotational speed of the engine or the corrected, rotational speed. The corrected rota: tional speed is equal, as is well khown'in the art, to, the. actual, rotational speed divided by the square root of the absolute temperature at inlet to the ehgine.

This invention is concerned with multi-stage. axial: flow' compressors haying, adjustable' pitch statorblades and has for an object to providean improved methodof controlling the flow conditions in such a compressor so as to, maintain a high'efiiciency of compression through a wide speed rangef Another object of'the intention is to, rqvide 'an improved method of, and apparatus for cpn t rplling the pitch of the stator blades. The. term stator blades includes i let, whirl vanes for an axialflow compressor which ay in, certain cases be, followed by another row of stationary blades;

Accordi l h it oa c nsi s n one aspect in a. ditions in a multi et od oi] onso n the flaw. stage al-tha masses f has e a r' is ea" ss ste axial velocity is higher th'anthe:

appropriate action of springs '17, and thereby to 2,705,590 Bate te Arr: 5,.

able pitch stator, blades, which, method comprises, over part at least of the range of. rotational speeds of the compressor, automatically varying the pitch of the, ad justablerpitch stator blades so that for each speed: in the range over which control is efiected the stator blades have an appropriate pitch. Thus, as the speed of the compressor progressively changes through therangefof rotational speeds in which the pitch of-the 'statorblades. varied, the pitch of the bla'deswill be automatically. and progressively. changed to have at' each speed. a value" to give. eificient operation. of the compressor at that speed.

According to this, invention in another aspect. there. is. provided the, combination with a'multi-stage axialrflow' compressor having adjustable-pitch stator blades, of

- adjustable-pitch stator blades comprises Pressure-sensitive means arranged to control the pitch of a ring of the adjustable-pitch stator blades which pressuresensitive, means is arranged to be loaded in one sense by a fluid pressure which is a predetermined function of: the corn pressor rotational speed and in the opposite sense byfia force which is a function. of the pitch of saidstator blades, so, that for each rotational speed of the compressor, the stator blades have a corresponding pitch.

Some constructional arrangements of. this invention as applied in an axial-flow compressor. of a. gas-turbine engine will not be described with reference, to the acco i n panying drawings in which:

Figure 1 illustrates diagrammatically a gas-turbine en; gine having an axial-flow compressor with adjustable stator blades;

Figure 2 is a detail section through a construction of gpe ns m han sm for d us n t e ad stab e tato lades;

Figure 3 illustrates a modification of the mechapism of; Figure 2;

Figure 4 illustrates another construction mechanism for the stator blades; and

Figu'r'e'S illustrates yet another construction of operatingmechanism fo'rt'he stator blades. h i i Referring to the drawings there is shown a gas-turbine at er ting engine(F 1gure 1) which comprises a multi-stage' axialflow. compressor 10, combustion equipmentdl, receiving compressed air from the compressor 10, and turbine 12 receiving combustion products" from the combustion equipmet 1 1, and arranged tofdrive' the compre sdriro; thi'ongha shaft 13,;

Fuel is supplied to the, combustion equipment 11 to be burnt in the, compressed air, by means of, a well known fuel system comprising an engine-driven slvfSh: plate] variahle-delivery pump 14. (Figures '1 'ar1d1'2') which comprises a I 4 e inders or. bores formed therein containing, plungersld which on rotation of the pump" rotor 15 are, constrained to reciprocateinf the, cylinders or bores against the draw 'infuel froth the. suctionpipe' '18 of the pump 14 and'to 'deliver itto. the engine through a delivery pipeflfil The pump drive from the engine is indicated a620, (Figure I).

The pump rotqr is formed with a centraYbore'Zl leading. from the suction pipe 18 of the I i m l9 series ofradial 'drillings ZZ vvh'ich place thefcentral' bore Zl in communication with a chamber. 23 housflg the 'pump rotor. 15. The pump rotor 15 thereby acts as a centrifugal pump and the pressure within the chamber 23 is a function of theengine rotational speed'ancil, thus of the rotational speed of the compressor 10,.

The multi-stage axial-flow compressor 10, is provided with one 'or more rings of"adjustable stator blades (illustrated as tworings of such blades 2 th, 24th,)" the Conveniently each adjustable blade may be provided with an operating arm engaged in a notched ring 26 rotatable in the stator structure of the compressor to rock the arms 25 of a ring of blades simultaneously.

In order that the compressor may operate efficiently over a wide range of engine speeds, it is desirable that the pitch of the stator blades 24a, 24b should be adjusted so that for each rotational speed of the compressor 10 their pitch is. suitable to give a high efliciency of the compressor. It is thus arranged that as the engine rotational speed increases so the pitch of the stator blades 24a, 24b changes proportionally and for this purpose the following control arrangement for the adjustable pitch stator blades may be provided.

One adjustable stator blade of each of the rings of blades 24a, 24b is connected for adjustment with an associated hydraulic ram 30 (Figures 1 and 2) and the hydraulic ram comprises a cylinder 31, an operating piston 32 working within the cylinder 31, and a control mechanism 33 whereby the position of the piston 32 within the cylinder 31 is determined by the engine rotational speed. Since the position of the piston 32 within the cylinder 31 determines the pitch of the associated stator blades 24a, 24b, the pitch of the stator blades 24a, 24b will thus be determined in accordance with the engine rotational speed.

The ram cylinder 31 is provided internally with a pair of bushes 34, 35 engaged by the stem 32:: of the piston so that the piston stem is guided in the bushes, and the piston 32 itself is located intermediate the length of the stem 32a and between the bushes 34, 35, so that the bushes act as limit stops for the piston. The stem 32a is hollow and in communication through bore 36 with the cylinder chamber 37 on one side of the piston head. In this way the eflective area of the side or end of the piston 32 exposed to chamber 37 is substantially greater than the side or end of the piston 32 exposed to cylinder chamber 38. The cylinder 31 is provided with a pressure fluid supply connection 39, which in this arrangement is connected for convenience to the delivery pipe 19 of the engine-driven swashplate type variable-delivery pump 14, and bores 40, 41 are taken from this connection 39 to the chambers 37, 38, on each side of the ram piston 32, the bore 40 to the chamber 37 on the side of the piston of larger effective area comprising a flow restrictor 42 the purpose of which will be clear from the following description.

Mounted on the end of the ram cylinder 31 adjacent the side of the piston 32 having the larger eifective area, there is provided the control mechanism 33 for the piston 32. This control mechanism comprises a chamber divided into two compartments 43, 44 by a flexible diaphragm 45, the compartment 4-3 which is nearer the ram cylinder 31 being connected to the suction pipe 18 of the fuel pump 14 through a connection 46 and the compartment 44 being connected by a pipe connection 47 to the chamber 23 of the fuel pump 14 into which pressure fluid is delivered by the centrifugal pump formed by the pump rotor. The flexible diaphragm is thus loaded in one direction (in a direction towards the ram cylinder) by a pressure which is a function of the engine actual rotational speed.

The flexible diaphragm 45 is also arranged to be loaded by a spring 50 accommodated within the hollow stem 32a of the ram piston 32 and for this purpose the end wall of the ram cylinder is formed with an axially-directed neck 51 having therein a bore fitted with a sleeve 52 containing a sliding push rod 53, one end of which bears on an abutment 45a on the diaphragm 45' and the other end of which bears against an abutment member 54 for the spring 55. The other abutment for the spring is formed integrally with the piston stem. The push rod 53 slides in the sleeve 52 and suitable fluid-tight seals, conveniently formed by lands 5311, are provided to prevent leakage of pressure fluid from the ram cylinder 31 into the compart ment 43 to the left of the diaphragm 45 as viewed in the drawings. It will be clear that as the piston 32 moves in the ram cylinder 31 towards the control mechanism 33, the load on the diaphragm 45 due to the spring 50 will increase and the load will be dependent upon the position of the piston 32 within the ram cylinder 31. It will also be clear that the spring load will oppose the fluid pressure load on the diaphragm 45.

The flexible diaphragm 45 is also arranged in this construction to be loaded by a secondary spring 55 in the same sense as by the spring 50 accommodated in the stem 32a of the piston 32. The secondary spring 55 has one abutment afforded by the abutment member 45a carried by the diaphragm 45 and its second abutment on a shoulder 56 formed within the neck 51 supporting the sleeve 52 for the push rod 53.

Movements of the flexible diaphragm 45 under control of the fluid pressure and spring loads are communicated to' a second push rod 57 mounted slidably in a bush 58 fitted in the wall of the diaphragm compartment 44 and movements of the push rod 57 are in turn communicated to a half-ball carrier 59 forming part of a valve mechanism. This mechanism comprises a half-ball element 62 which co-operates with a bleed port 63 in member 64 tocontrol the flow of pressure fluid from a passage 60, which communicates with the ram cylinder chamber 37, into a chamber 61 housing the valve mechanism. The second push rod 57 is formed with lands 57a co-operating with the wall of the bore of bush 53 to provide a fluid seal between the diaphragm compartment 44 and the chamber 61. This chamber 61 is connected by pipe connection 65 with the suction pipe 18 of the fuel delivery pump 14. The half-ball carrier 59 and the second push rod 57 are lightly loaded by a spring 66 towards engagement with the flexible diaphragm 45.

The operation of the hydraulic ram is as follows.

Assuming that the engine is stationary then the halfball bleed valve 62 will be closed on its seat around port 63 and the piston 32 will be at the end of the ram cylinder 31 remote from the control mechanism 33 where it is held by the main spring 50. It now the engine is started up the pressure on each side of the ram piston will increase, but since there is no leak from cylinder chamber 37, the piston 32 will remain stationary. At the same time, the fluid pressure in pump chamber 23 and thus the fluid pressure acting on the diaphragm 45 will increase due to increase in engine speed.

When the fluid pressure acting on the diaphragm 45 has increased sufliciently to overcome the secondaryspring 55 and thereby to permit the half-ball bleed valve 62 to open, pressure fluid bleeds ofl from the ram cylinder chamber 37 on the side of the piston having the greater eflective area and the pressure within the cylinder on this side of the piston head will fall due to the presence of the flow restriction 42 in its feed bore 40. When the pressure on this side of the cylinder falls sufliciently;

the resultant pressure load on the piston 32 will overcome the spring 50 and the piston 32 will start to move within the cylinder 31 and will continue to move so long as the engine rotation speed increases until the piston head 32 abuts against the limit stop formed by bush 35.

It will be seen that the secondary spring 55 acts to aliord a lower limit to the range of speeds over which the ram 30 is effective to adjust the pitch of the stator blodes 24a, 24b.

Assuming now that the engine speed is steady within the range of speed over which the ram 30 operates to adjust the pitch of the stator blades 24a, 24b and that the engine speed is increased to a new speed in this range, then the pressure acting on the diaphragm 45 will be increased permitting the half-ball bleed valve 62 to open and to bleed off fluid through passage from ram cylinder chamber 37 communicating with the side of the piston 3270f larger effective area. The pressure on this side of the piston 32 therefore falls (due to the presence of flow restriction 42 in its feed bore 43) and the piston 32 moves to the right as seen in the drawing within the ram cylinder 31, gradually increasing the spring load aflorded bythe main spring 50 on the diaphragm 45, thus tending to close the half-ball bleed valve 62 and to cut down the bleed flow from the ram cylinder chamber '37. When the piston 32 reaches a position in the cylinder appropriate to the new engine speed, the half-ball bleed valve 62 will have been closed off to such an extent that the pressure within the cylinder space 37 commun1-.

eating through passage 60 with the bleed valve 62 has such a value that the loads acting on the piston 32 are balanced and the piston will stabilise in this position. In this position moreover the spring load on the diaphragm 45 will balance the fluid pressure load derived from chamber 23 due to the pressure from the pump rotor.

From. the foregoing: description, it will: be seen that for each. engine actual: rotational speed, the piston 32: occupies: a corresponding: position inwthe: ram. cylinder so that; the pitch offthestator: blades 24a; 24b controlled;

but may be loaded by an arrangement such as is shown.

in: Figure.- 3. In this. arrangement; a. pipe 1 701 is provided with: its ends connectedato.thesuctionrpipe 18* and there is. arranged in the pipe. 70" afixed-capacity positivedisplacement pump-.74 which. is driven at a. speed. propor-. tional to engine speed; and, hydraulically in seriesthere. with, a.restricting.- orifice. device: 71 whereof the effective area; is. controlled: by a,- value; element 72' carried by an expansible; capsule 731 arranged to expand? and. contract inresponseto. variations in the engine intake temperature. The pipe connection 47 from the compartment 44 of the, diaphragm chamber of! the ram: control; mechanism. 33 is connected: to; the pipe. 70L between the orifice 71 and: the delivery of'pump. 7.4:. and the pipe connection 46 from. the compartment 4-3"on the. other side. of the diaphragm, is. connected: to pipe. 70. downstream oh the orifice 71 so that the. diaphragm 45. is; loaded by thepressure. drop. across orifice 71 The. fuelflow. through. the orifice 7:1 is proportional: to engine speed, and theetfective area of theorifice 7 1 is controlled by capsule 73'. to be. proportional-to the square root: of the. engine inlet: temperature, the pressure: drop. across orifice 71 and thus the pressure d iiferenceapplied across. the. diaphragm 45 is proportional tor the corrected rotational speed (:that is. the actual engineQrotational speed divided by the. square. root of the inlet temperature.)

The. capsule 7:3 is. illustrated as being connected internally by capillary 75. to. a bulb. 76 located in; the, engine, intake (see also: Figure 1). and the. bulb 76, capillary 75v and capsule 73 are all filled with fluid so that the capsule l5 expands and contracts with thefluid filling on sensing by; the bulb. 716: of increases; or. decreases. Off the air temperature. in the. engine intake. The ram 3.0.- is supplied with pressure fluid for its; operation as. in Figure 2 by.

being. connected through the: supply connectiorr' 39, to. the 5 delivery pipe 19 ofi themain fueljpump 14.

In a further constructional arrangement illustrated in Figure 4. of a ram device 30. suitablefor use in accordancewith the invention, a control force is obtained'which is! proportional to. the. square of the rotational speed, and opposing force is obtained which: is proportional to the engine inlet temperature, and: these forces are. suitably combined to provide a resultant force approximately proportional. to. theengine corrected speed. In this arrangement the compartment. 43. of the chamber containing the diaphragm 45; is connected to the suction side of the fuel pump 14 as in the arrangement of Figure 2 and the other compartment 44. is. connected as in Figure 2 to the chamber 23. of the fuel pump. 14. into. which pressure fluid is delivered by the centrifugal pump. formed in the pump rotor.

There is provided in. the compartment- 43 nearer the ram cylinder 31 an. arrangement of capsules to give the desired control force, dependent on the inlet air temperature.

The push rod. 53, of which. one end bears on the diaphragm 45' and the other end bears. against the abutment member 54 for the compression spring of the ram piston, is provided with a radially extending disc-like part 80. Between said disc-like part 80 and the wall of the compartment 43 nearer the ram cylinder 31, there is arranged a capsule 81, the interior of which communicates by means of the capillary tube with a temperature-sensitive bulb element 76 in the engine air intake. The interior of the capsule 81, the capillary 75 and the bulb element 76 are filled with a fluid which expands on increase in temperature, in a known manner. The capsule 81 is normally in contact with the disc-like part and the wall of the compartment 43. Between the disclike part 80 and flange-like abutment 82 which projects radially inwardly from the wall of the compartment" 43" between the diaphragm and: the disc-like part, there islocated an evacuated. capsule 83; This capsule-83 is normally in contact with the disc-like part 80' and. with the abutment 82; Theflange-like abutment! 82; is so arranged that the full area of the diaphragm 45 is still subjected on the side thereof remote from the ram cylinder 31 ,"t-he spring 84 abutting the member 45a carried by the diaphragm 45 and the wall of compartment 44 remote'fir'om the ram cylinder 31. This spring 84 serves-to ccunteract the resilient load exerted by the capsules 8'1 83. In all other respects this constructional arrangementis;similar-to.

that described above withreference-to-Figure 2.

In operation, the diaphragm 45 is loaded in thesense of opening the half-ball valve 62 by the pressure fluid from the pump chamber 23: This controls load is, therefore, proportional to the square of the engine rotational speed. The opposing load is produced as follows:- an; increase in thetemperature of the air at the inlet of the engine causes the fluid in the parts 7.5, 75, 81 toexpand and causes a load to be supplied to the push-rod 5 3, in

the sense of closing the half-ball valve 62, and to move.

the diaphragm against the spring 84. which abuts it. This load is arranged to be proportional to the temperature of the air at the inlet of theengine. It will be appreciated that the provision of the evacuated capsule 83 on the side of the disc remote from the fluid-filled capsule $1 ensures, by correct selection of the relativeareasof the two capsules 81, 83, that the load applied toin the compartment 43;

To produce a control force approximately proportional to engine corrected speed in such a system, the dimensions of the capsules 81 83 and of the diaphragm 45- are so arranged that, at one selected value of the'corrected speed, the stator blades 24a, 24b willhave one unique the disc 80 is independent of the fuel pump inlet pressure setting, irrespective of the actual temperature. In this way, it may be arranged that any error which is introduced as the corrected speed departs from this selected value, the error being dependenton the intake temperature and on the relation ofthe stator hlade pitch to the selected pitch, will be negligible over the range of control required in practice.

In another constructional arrangement of ram device, a further modified form of temperature control is provided. In this case (Figure 5 the half-ball valve 62 is; 1 carried on one end of a, lever 98 which rocks on a pivot formed by a flexible diaphragm 91 separating chamber 44 from chamber 61. The portion ofi the lever on the side of the diaphragm 91 remote from the half-ball valve 62 is connected by a tappet element 93 to. the flexible diaphragm 45, so that movement of the diaphragm 45 in a direction away from the ram cylinder31 tendsto allow the half-ball valve 62 to close. There is alsoprovided a spring 94 serving to urge thehalfiball valve towards the closed position. The spring 94 is accommodated in a chamber 95 and is arranged to bear through a push-rod 96 on the end of the lever 90 adjacent to t-he half-ball valve 62. The end of the spring 914 reniotefrom the half-ball valve is arranged to abut a second push-rod 97 which is loaded by means of a capsule 173' which has its interior connected by capillary tube 75' to a tempera ture-sensitive element such as the element 76 in the engine air intake, and which capsule 173 is arranged to expand on increase of temperature. A load proportional to temperature is thereby applied to the half-hall valve 62 in a sense of tending to close the half-hall valve (52,"the load increasing on increase of temperature and being applied by means of the capsule 173 through the spring 94. In other respects this arrangement is similar to thiej second arrangement described.

It will be seen that in this arrangement the half-ball valve is loaded through diaphragm 45 in the sense of opening by a force, due to the pressure fluid of the centrifugal pump of the engine fuel pump, proportional to the square of engine speed, and in the opposed sense by a force proportional to the inlet temperature of the engine.

By suitable selection of the areas of the diaphragm 45 and of the capsule 173, and of the moment arms of the lever 90, a control force may be produced on the bleed valve 62, as in the second arrangement described, approximately proportional to engine corrected speed.

In this construction, no secondary spring such as the spring 55, is provided for pre-loading the diaphragm 45, but the spring 94 can be arranged to have the same efiect as the spring 55.

We claim:

1. A multi-stage axial-flow compressor having at least one ring of adjustable-pitch stator blades connected for simultaneous adjustment, power means operatively connected to said blades for adjusting the pitch of said blades, pressure-sensitive means, means to produce a fluid pressure which is a predetermined function of the compressor rotational speed and to apply said fluid pressure to the pressure-sensitive means to load it in one sense, and means to load said pressure-sensitive means in the opposite sense with a load which is a function of the pitch of the adjustable pitch stator blades, said pressure-sensitive means being operatively connected to said power means for adjustment of said power means when the balance of the loads to which said pressure-sensitive means is subjected is upset, said adjustment being in the sense to restore said balance, whereby for each rotational speed of tge compressor the stator blades have a corresponding pitc 2. A multi-stage axial-flow compressor as claimed in claim 1, wherein said power means comprises a hydraulic ram having a cylinder and a piston, said piston having ends provided with eflective areas of diflerent size, connecting means between said piston and said stator blades for adjusting simultaneously the pitch of the stator blades of a ring of adjustable stator blades, a fluid pressure source, pressure connections from said source to said ram to load both ends of said piston, said pressure-sensitive means controlling the fluid pressure on the end of the piston of larger eflective area to have a value relative to the fluid pressure on the smaller area end of the piston to balance the loads on the piston, whereby for each rotational speed in the range of rotational speeds of the compressor the ram piston occupies a corresponding posi-;

tion in the ram cylinder.

3. A multi-stage axial-flow compressor having at least one ring of adjustable-pitch stator blades, the blades of said ring being connected for simultaneous adjustment; a hydraulic ram including a cylinder, and a piston in said cylinder dividing said cylinder into first and second pressure spaces and connected with the stator blades of said ring to adjust the pitch of said blades progressively on progressive movement of said piston in said cylinder, said piston having its side which isopen to the first pressure space of less effective area than its side open to the second pressure space; a pressure fluid source; a first supply connection leading from said pressure fluid source to said first pressure space and being in continuous communication therewith; a second supply connection from said pressure fluid source to said second pressure space; a flow restrictor in said second supply connection; a bleed conduit connected with said second pressure space; a bleed valve controlling the outlet of said bleed conduit; spring means arranged to load said bleed valve towards the closed position; means to vary the load exerted by said spring means on said bleed valve in dependence on the position of said piston in said cylinder; a second pressure fluid source having a fluid pressure which is a function of the compressor rotational speed; and a pressure-responsive device connected to be subjected to the fluid pressure of said second pressure fluid source and connected to said spring means to oppose the load exerted thereby.

4. A multi-stage axial-flow compressor as claimed in claim 3, comprising also a second spring means arranged to apply a preselected substantially constant load to the pressure-responsive device in the sense of closing the bleed valve, thereby to determine the lower limit of compressor rotational speed at which adjustment of the stator blade pitch is efiected.

5. A multi-Stage axialflow compressor as claimedin claim 4, having a first abutment for one end of said second spring means, theopposite end of said second spring means being in abutment on the pressure-sensitive device.

6, A multi-stage axial-flow compressor as claimed in claim 3, comprising as said second pressure fluid source a centrifugal pump driven at a speed proportional to the compression rotational speed and connected to said pressure-sensitive device to load the pressure-responsive devicedas a function of the actual compressor rotational spee 7. An arrangement as claimed in claim 6, having the centrifugal pump formed in the rotor of a swashplate variable delivery pump forming part of an engine fuel system.

8. A multi-stage axial-flow compressor as claimed in claim 3, comprising as said second pressure fluid supply source fixed-capacity positive-displacement pump arranged to be driven at a speed proportional to the compressor rotational speed, a delivery conduit from said pump, a flow-restricting orifice arranged in said conduit to restrict flow therethrough, and pressure connections from said conduit on each side of said flow-restricting grifice to corresponding sides of said pressure-responsive evice.

9. A multi-stage axial-flow compressor as claimed in claim 8, comprising also temperature-responsive means arranged to respond to the temperature at the inlet of the compressor and to vary the effective area of said flowrestricting orifice to be proportional to the square root of the temperature at the inlet to the compressor.

10. A multi-stage axial-flow compressor as claimed in claim 3, having said second pressure fluid supply source arranged to apply to said pressure-responsive device a control force which is proportional to the square of the compressor rotational speed, and comprising also means responsive to the temperature at the inlet to the compressor and arranged to load said pressure-responsive device in a direction opposite to said control force with a force which is proportional to said temperature, said pres sure-responsive device and temperature-sensitive means being so arranged that at one selected value of the corrected speed of the compressor the ram piston will occupy a given setting irrespective of the value of said temperature.

11. A multi-stage axial-flow compressor as claimed in claim 3, comprising also temperature-sensitive means arranged at the intake of the compressor, and arranged to load said bleed valve in a sense of closure with a load which is dependent on the temperature at the intake to the compressor, and having said second pressure fluid supply source arranged to load the pressure-responsive device with a load proportional to the square of the compressor rotational speed, said temperature-sensitive means and said pressure-responsive device being so arranged that the control force acting on the bleed valve is approximately proportional to the compressor corrected speed.

References Cited in the file of this patent UNITED STATES PATENTS 2,147,196 Fox Feb. 14, 1939 2,371,706 Planiol Mar. 20, 1945 2,429,005 Watson et a1. Oct. 14, 1947 2,477,954 Blane Aug. 2, 1949 2,481,334 Nicolls Sept. 6, 1949 2,545,703 Orr Mar. 20, 1951 2,613,029 Wilde Oct. 7, 1952 FOREIGN PATENTS 918,317 France Oct. 14, 1946 967,212 France Mar. 22, 1950 

